The present invention relates to a sheet feeding device for a printer, particularly a stencil printer including an ink drum with a master wrapped therearound and a press drum one of which is pressed against the other during printing.
A thermal digital stencil printer or similar printer extensively used today includes an ink drum capable of rotating at a variable speed in accordance with an input print speed with a master wrapped therearound. A press roller, or pressing means, is pressed against the ink drum relatively to the ink drum with the intermediary of a sheet. A pair of registration rollers, or registering means, feed the leading edge of a sheet toward a print position between the ink drum and the press roller. A pair of separation rollers and a pickup roller, or sheet feeding means, pay out the leading edge of a sheet toward the registration rollers. The sheet abuts against the registration rollers and forms a loop. Japanese Patent Laid-Open Publication No. 8-59031, for example, discloses a sheet feeding device applicable to the above printer.
Japanese Patent Laid-Open Publication No. 6-40137, for example, teaches a sheet feeding device applicable to a stencil printer of the type including a sheet bank in addition to the ink drum, press roller, registration rollers, and sheet feeding means. The sheet bank is positioned below the print position for feeding a sheet toward the registration rollers. A pair of intermediate rollers, or sheet conveying means, convey the leading edge of a sheet paid out from the sheet bank and cause it to abut against the registration in rollers and form a loop. This sheet feeding device includes a plurality of trays loaded with sheets and allows the operator to pull out desired one of the trays for replenishing sheets, standing at the front of the printer (front loading system).
In any one of the conventional printers, a main motor capable of rotating at a variable speed in accordance with the input print speed drives the ink drum. Specifically, a print speed key, or print speed setting means, is positioned on, e.g., an operation panel for allowing the operator to input desired one of a plurality of print speeds. At the time of printing, the rotation speed of the ink drum is varied in accordance with the print speed input on the print speed key (set print speed hereinafter).
The press roller may be replaced with a press drum having substantially the same diameter as the ink drum and including a sheet clamper. The press drum is rotatable at substantially the same peripheral speed as the ink drum in the opposite direction to the ink drum with the sheet damper retaining the leading edge of a sheet, so that the leading edge of the sheet can be forcibly separated from the ink drum. With the press drum, it is possible to prevent the sheet from rolling up without being separated from the ink drum, to reduce noise, and to enhance the positional accuracy of an image on a sheet in the direction of sheet conveyance (registration accuracy hereinafter).
In the printer taught in the above Laid-Open Publication No. 8-59031, the main motor drives the separation rollers and pickup roller via a belt, gears and clutches or cams. This kind of driveline is undesirable because the peripheral speed of the separation rollers and that of the pickup roller are dependent on a print speed varying every moment in accordance with the set print speed or, e.g., the extension of the belt due to aging, backlashes of the gears, and so forth. As a result, the amount of the loop varies from one print speed to another print speed. It follows that particularly when the print speed is low, the slip of the sheet on the separation rollers and pickup roller is aggravated, making the amount of feed short. The short amount of feed causes the sheet to skew or prevents it from being fed. Further, when the print speed is high, the sheet produces noise when straightened, i.e., when its loop disappears because the lower separation roller does not rotate and increases the load on sheet conveyance.
On the other hand, in the printer proposed in Laid-Open Publication No. 6-40137, the trays of the sheet bank each are provided with a respective separation roller, a pickup roller and other rollers for conveyance. A sheet feed motor implemented by a stepping motor drives such rollers of the sheet bank. The sheet feed motor is so controlled as to increase the sheet conveying speed of, e.g., the separation roller by 0% to 25% in accordance with five consecutive print speeds by way of example, taking account of, e.g., the slip of a sheet being conveyed toward the registration rollers. However, when the sheet conveying speed of, e.g., the separation roller is simply increased, a stable loop is not achievable. Therefore, at a low print speed, the slip of the sheet on the separation roller and other rollers is aggravated and makes the amount of feed short, resulting in the skew or the feed failure of the sheet.
At a high print speed, a sheet forms an excessive loop and produces noise when the loop disappears because the separation pad is fixed and increases the load on sheet conveyance. Moreover, as the above document shows in FIG. 1, transport paths extending from the top and bottom trays of the sheet bank are sharply curved. Therefore, when the print speed is high, the sheet produces noise when sliding on guides forming the above transport paths. The above document therefore does not set a sheet conveying speed in consideration of noise of the entire printer.
The conventional sheet bank does not include registering means independent of the registration rollers of the printer body for conveying the leading edge of a sheet toward the registration rollers at a preselected timing. The present invention applies the above problems discussed in relation to the formation of a loop to the exclusive registering means of the bank also.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Laid-Open Publication No. 9-216448, Japanese Patent Publication No. 5-32296, Japanese Utility Model Publication No. 5-18342, Japanese Patent Laid-Open Publication Nos. 9-30714, 5-124737, 5-221536, 6-40137, 6-144600, 7-137851, 2-265825 and 10-35911, and U.S. patent application Ser. No. 09/025,037 which issued as U.S. Pat. No. 6,098,536 and Ser. No. 09/042,615 which issued as U.S. Pat. No. 5,931,090.